Some one who eavesdrops and then joins or interrupts secretive or private conversations without your, or your groups invitation.

Person 1 : "hey did you hear about Sally?"
Person 2 : " I herd she....."
Social Sniper (from the other side of the room): "Pissed her pants when she saw that spider, yeah bro it was awesome!"
Person 1 & 2 : we just got sniped.... f`in social sniper